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Industry structure
The American Petroleum Institute divides the petroleum
industry into five sectors:
 upstream (exploration, development and production of
crude oil or natural gas)
 downstream (oil tankers, refiners, retailers and consumers)
 pipeline
 marine
 service and supply
Oil companies used to be classified by sales as
"supermajors" (BP, Chevron, ExxonMobil, ConocoPhillips, Shell,
Eni and Total S.A.), "majors", and "independents" or "jobbers". In
recent years however, National Oil Companies (NOC, as opposed
to IOC, International Oil Companies) have come to control the
rights over the largest oil reserves; by this measure the top ten
companies all are NOC. The following table shows the ten largest
national oil companies ranked by reserves and by production.
Most upstream work in the oil field or on an oil well is
contracted out to drilling contractors and oil field service
companies.
Midstream operations are sometimes classified within the
upstream sector, but these operations compose a separate and
discrete sector of the petroleum industry. Midstream operations
and processes include the following:
Gathering. The gathering process employs narrow, low-
pressure pipelines to connect oil- and gas-producing wells to
larger, long-haul pipelines or processing facilities.
Processing/refining. Processing and refining operations
turn crude oil and gas into marketable products. In the case of
crude oil, these products include heating oil, gasoline for use in
vehicles, jet fuel, and diesel oil. Oil refining processes include
distillation, vacuum distillation, catalytic reforming, catalytic
cracking, alkylation, isomerization and hydrotreating. Natural gas
processing includes compression; glycol dehydration; amine
treating; separating the product into pipeline-quality natural gas
and a stream of mixed natural gas liquids; and fractionation, which
separates the stream of mixed natural gas liquids into its

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